With complex-valued, multi-echo data (i.e., ME-EPI with both magnitude and phase information reconstructed), it is possible to estimate and correct for distortion on a volume-by-volume basis. I'm definitely not an expert in this area, but recently I've been reading a bit about it and there is a method called DOCMA that uses the phase data from the first two echoes to compute a phase-difference map and perform distortion correction on each volume of a time series, independently. The math definitely goes over my head, but it looks like the method is roughly equivalent to simply splitting the time series up into individual volumes and then performing standard distortion correction using the first two echoes as the field map. If that is the case, then I think this could be implemented fairly easily once #30 is merged.

Other methods for dynamic distortion correction that I've come across, including the UMPIRE multi-echo phase unwrapping method, seem to have special requirements, like uneven, highly specific echo times or alternating echo times for a single-echo protocol. DOCMA, on the other hand, seems to be applicable to any multi-echo sequence with phase information.

EDIT: After further investigation, I don't think the phasediff workflow will work as needed for dynamic distortion correction because the skullstripping performed in the workflow seems to be static (i.e., it treats 4D data as 3D and only produces one 3D mask). The whole procedure will probably require a splitting node wrapping around the workflow to separate all of the 4D datasets into 3D volumes before calculating and applying the field maps.